Prolonged contact between body parts and a mattress surface tends to put pressure onto the reclining person's skin. The pressure tends to be greatest on the body's bony protrusions (such as sacrum, hips and heels) where body tissues compress against the mattress surface. Higher compression tends to restrict capillary blood flow, called “ischemic pressure”, which causes discomfort. The ischemic pressure threshold normally is considered to be approximately 40 mmHg. Above this pressure, prolonged capillary blood flow restriction may cause red spots or sores to form on the skin (i.e., “stage I pressure ulcers”), which are precursors to more severe tissue damage (i.e., “stage IV pressure ulcers” or “bed sores”). The preferred pressure against the skin of a person in bed remains generally below the ischemic threshold (e.g., below 40 mmHg, preferably below 30 mmHg).
Pressure build up from contact with the fabric ticking or outer fabric cover of a mattress may be more acute for heavy weight people who tend to sink farther into a mattress and stretch the ticking or cover to a greater extent. This is called “hammocking”, which is to be avoided. See U.S. Pat. Nos. 5,655,241 (Higgins) and 5,475,881 (Higgins).
Poor body alignment on a mattress also leads to body discomfort, leading to frequent body movement or adjustment during sleeping and a poor night's sleep. Particular challenges are faced when a reclining person weighs 350 pounds or more. Higher weight persons tend to sink farther into and depress a mattress more than lower weight persons. Higher weight persons may cause the mattress to sag excessively or bottom out, particularly at the sacrum supporting region. A sagging mattress also allows the person's waist to drop relative to the rib cage and hips, and causes stress to muscles, tendons and ligaments. Such stress may lead to joint pain, particularly lumbar and back pain.
An ideal mattress has a resiliency over the length of the body reclining thereon to support the person in spinal alignment and without allowing any body part to bottom out. A preferred side-lying spinal alignment of a person on a mattress maintains the spine in a generally straight line and on the same center line as the legs and head. An ideal mattress further has a low surface body pressure over all or most parts of the body in contact with the mattress. This objective, however, competes with the objective of providing satisfactory support for a heavy weight person.
Hospitals and healthcare providers continue to seek lower cost alternatives for mattresses that may be used for patient beds. Mattress constructions with springs and heavy supporting structures that may be appropriate for home use are not appropriate for hospitals and clinics. Patient-supporting mattresses generally should be lighter weight and portable so that they can be moved with the patient. In some cases, such mattresses are disposable. These objectives, however, compete with the objective of providing satisfactory support for a heavy weight person.
Numerous mattress constructions have been proposed to vary the body support without incorporating traditional springs. For example, U.S. Pat. No. 7,036,172 (Torbet, et al.) discloses several mattress constructions having multiple foam layers of different densities positioned in different sections to vary the supporting characteristic in each section. In some embodiments, Torbet, et al. has a single foam layer in the shoulder and hip supporting portion, and punches holes of varying depths into the foam surface to vary the support characteristic.
U.S. Pat. No. 5,749,111 (Pearce) shows seat cushions and mattresses with a base material of a gelatinous elastomer that is molded to form a plurality of hollow columns. The hollow columns buckle under applied loads. Open or closed cell foam can be held within the hollow columns to increase the firmness of the cushions. U.S. Pat. No. 7,076,822 (Pearce 2) includes a layer with hollow columns formed therethrough in a mattress construction.
In addition to alternative mattress constructions, mattress pads or overlays to dispose over a surface of an existing hospital mattress to reduce pressure on a reclining patient are known. U.S. Pat. Nos. 5,201,780; 5,255,404; and 5,303,436 to Dinsmoor, III, et al. show anti-decubing mattress pads that include foam support columns that are hollowed out to varying degrees to form conical cavities of different depths to vary the support or spring performance of the foam support columns. Such pads or overlays add additional cost to patient care.
There are an increasing number of people weighing 350 pounds or more, and in some cases up to 1000 pounds. The bedding industry, and particularly the medical mattress industry, continues to seek alternative mattress constructions that can adequately support such heavy weight persons, yet still meet the competing objectives of low cost, portability, satisfactory body support and low surface body pressure.